Subject: RE: [mobile-ip] RFC3012bis - Comments from ADs (Protocol related) (ISSUE 13)

From: "Ahmad Muhanna" <[email-masked]>
Date: Thu, 26 Sep 2002 11:18:54 -0500

please see comments inline.

Regards;
Ahmad Muhanna

Non-editorial comments associated with RFC3012bis:

Please post your comments with the appropriate issue number in the subject
line (instead of simply replying to this email).

-Chairs.

13. Section 8
    The concatentation of fields for MD5 include "length" and "SPI"
    but it isn't clear to me how many bits of length and SPI are fed
    in since these two fields are not fields in the protocol itself as
    I understand it.

If we read the whole thing it becomes clearer.
"

   To compute the authenticator, apply MD5 [10] computed on the
   following data, in the order shown:

      High-order byte from Challenge || Key ||
      MD5(Preceding Mobile IP data ||
      Type, Subtype (if present), Length, SPI) ||
      Least-order 237 bytes from Challenge

   where the Type, Length, SPI, and possibly Subtype, are the fields
   of the authentication extension in use.  
"

These are the fields of the authentication extension specified in section 5.
"
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Subtype    |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              SPI                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Authenticator ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 3: The Generalized Mobile IP Authentication Extension

      Type            36 (not skippable) (see [7])

      Subtype         a number assigned to identify the kind of
                      endpoints or the other characteristics of the
                      particular authentication strategy

      Length          4 plus the number of bytes in the Authenticator;
                      MUST be at least 20.

      SPI             Security Parameters Index

      Authenticator   The variable length Authenticator field

   In this document, only one subtype is defined:

      1               Mobile-AAA Authentication subtype (see section 6)
"

This tells us that the length field is two bytes. The SPI field is 4 bytes.

Regards,
Ahmad